This application is based on application No. H12-236231 filed in Japan, the content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a gas discharge display device used for image display for computers, televisions, and the like, and in particular to a surface discharge AC plasma display panel.
2. Related Art
In recent years, there have been high expectations for large-screen televisions with superior picture quality such as high-definition televisions. In the field of display devices, plasma display panels (hereafter referred to as PDPs) have become the focus of attention for their ability to produce large-screen slimline televisions, with sixty-inch models already having been developed.
PDPs can be roughly divided into direct current (DC) types and alternating current (AC) types. At present, AC types, which are suitable for large-screen use, are prevalent.
A typical surface discharge AC PDP is described hereafter. A front panel and a back panel are arranged in parallel to each other with barrier ribs interposed therebetween. Discharge gas is enclosed in a discharge space which is partitioned by the barrier ribs. Scan electrodes and sustain electrodes are aligned in parallel on the front panel, and a dielectric layer is formed on the front panel so as to cover the scan and sustain electrodes. Also, address electrodes and the barrier ribs are arranged on the back panel, and red phosphor layers, green phosphor layers, and blue phosphor layers are formed between the barrier ribs.
FIG. 13 shows an electrode matrix of this PDP. In the drawing, for example, the number n of scan lines L is 4, and the number m of address lines is 6.
Pairs of scan electrodes SC1-SC4 and sustain electrodes SU1-SU4 are arranged in parallel at a predetermined pitch, and address electrodes A1-A6 are aligned perpendicular to the scan and sustain electrodes. Discharge cells are formed at the points where the pairs of scan and sustain electrodes cross over the address electrodes. Adjacent discharge cells are separated by barrier ribs RIB1-RIB7.
To drive the PDP, drive circuits are used to apply pulses to the electrodes, which causes discharge and emission of ultraviolet light from the discharge gas. This ultraviolet light is absorbed by the particles of red, green, and blue phosphors in the phosphor layers, causing excited emission of light.
Discharge cells in an AC PDP are fundamentally only capable of two display states, ON and OFF. Accordingly, a field timesharing gradation display method is adopted whereby one field is divided into multiple sub-fields having predetermined weights and a gray scale is expressed by the combination of the sub-fields.
FIG. 14 shows a method of dividing one field when 256 gray levels are expressed. In the drawing, the horizontal direction represents time, and the areas filled in with black represent discharge sustain periods.
FIG. 15 shows an example of drive voltage waveforms which are applied to the electrodes in one sub-field, when driving the PDP according to the above method. As illustrated, one sub-field is made up of a write period, a sustain period, and an erase period.
In the write period, the sustain electrodes SU1-SUn are held at a fixed potential (0V in this example). A write pulse Pa is selectively applied to the address electrodes A1-Am according to image data to be displayed, while a scan pulse Pscn whose polarity is opposite to the write pulse Pa is applied to the scan electrodes SC1-SCn.
As a result, the potential difference between the scan and address electrodes causes first write discharge, which in turn causes second write discharge between the scan and sustain electrodes (hereafter the first write discharge and the second write discharge are collectively called xe2x80x9cwrite dischargexe2x80x9d). Hence a wall charge necessary for sustain discharge to occur is accumulated.
By performing such write discharge sequentially for the scan electrodes SC1-SCn, image data to be displayed is written.
In the sustain period, AC sustain pulses Psy and PSX are applied in bulk to the scan electrodes SC1-SCn and the sustain electrodes SU1-SUn. This causes sustain discharge to continuously occur in the discharge cells where the wall charge has been accumulated in the write period, as a result of which an image is displayed.
In the erase period, an erase pulse Pe is applied to all sustain electrodes SU1-SUn, to cause erase discharge. As a result, the wall charge which remains after the sustain discharge is mostly neutralized.
According to this drive method, since a great number of scan lines need to be scanned within the write period, the write discharge tends to become unstable. When the write discharge is unstable, the light emission caused by the subsequent sustain discharge becomes unstable, too.
This problem appears to be solved by setting the voltage of the write pulse at a high level. However, limitations in the performance of the data driver make it impossible to increase the voltage of the write pulse.
Accordingly, to produce an excellent image display, it is of particular importance to perform write discharge reliably within a write period.
Recently, various PDPs have been developed to improve panel brightness. Examples are a PDP whose filling pressure of discharge gas is set equal to or greater than an atmospheric pressure, and a PDP whose discharge gas contains Xe at a partial pressure of 10% or more. Such PDPs have particularly high write discharge firing voltages and so the problem of unstable write discharge is more serious. For this reason, it is difficult to drive these PDPs by the drive method shown in FIG. 15.
To overcome this problem, a drive method that introduces a set-up period before the write period is disclosed by Japanese Laid-Open Patent Application No. H08-212930.
FIG. 17 shows an example of drive voltage waveforms according to this method. As shown in the drawing, a set-up pulse Prn of positive polarity is applied to the scan electrodes SC1-SCn in the set-up period.
By such applying the set-up pulse of the rectangular wave, set-up discharge takes place and as a result the wall charge remaining in the discharge cells after the erase discharge is completely neutralized. Also, priming effects that assist the subsequent write discharge to occur easily and reliably are obtained. Thus, this method is effective to stabilize the write discharge, but the level of stabilization achieved solely by this method is still insufficient, and other solutions are desired too.
To stabilize the write discharge, Japanese Laid-Open Paten Application No. H06-289811 discloses a drive method that applies a base pulse whose polarity is opposite to a write pulse, to scan electrodes in a write period.
FIG. 16 shows an example of drive voltage waveforms according to this method. In the drawing, the positive write pulse Pa is applied to the address electrodes A1-Am. Also, a base pulse having a base voltage Vb of negative polarity and constant wave height is applied to the scan electrodes SC1-SCn, throughout the write period, and a negative scan pulse Psco is superimposed on the base pulse.
When the base pulse is applied to the scan electrodes in this way, the potential difference between the address and scan electrodes and the potential difference between the scan and sustain electrodes increase by the degree of the base pulse applied. This encourages the first write discharge and the second write discharge to occur more reliably. As a result, the write discharge takes place unfailingly with no need to increase the voltage of the write pulse, with it being possible to improve the picture quality.
This base pulse applying method can drive, with a certain measure of success, a PDP whose discharge gas filling pressure is equal to or greater than an atmospheric pressure and a PDP whose discharge gas contains Xe at a partial pressure of 10% or more.
Even in this method, however, if the absolute value of the base voltage Vb is set high, discharge errors are likely to occur at the beginning of the write period, which results in a drop in picture quality.
For example, in the case where the base pulse applying method is adopted to a PDP which is less prone to write discharge due to variations in manufacturing or the like (hereinafter such a PDP is referred to as having low dischargeability), the absolute value of the base voltage Vb need be set higher in order to increase the write voltage. This tends to cause discharge errors at the beginning of the write period, thereby deteriorating the picture quality.
It is thus desired to perform data writing reliably even for a PDP that requires a high write voltage.
The present invention aims to provide a gas discharge display device that can perform stable write operations on a gas discharge panel and thereby produce an image display of superior quality.
The stated object can be achieved by a gas discharge display device including: a gas discharge panel having a first substrate and a second substrate that are opposed to each other, a group of first electrodes and a group of second electrodes being arranged on a main surface of the first substrate which faces the second substrate, a group of third electrodes being arranged on a main surface of the second substrate which faces the first substrate so as to cross over the group of first electrodes and the group of second electrodes, and a discharge gas being enclosed in a gap between the first and second substrates; and a drive circuit which writes data in a write period, and sustains a discharge in a sustain period, wherein the drive circuit applies a scan pulse and a base pulse which is superimposed on the scan pulse, to the group of first electrodes in the write period, and a voltage of the base pulse varies at an average rate of no greater than 10V/xcexcsec, during a first period from when the application of the base pulse starts until immediately before the application of the scan pulse starts.
With this construction, image data is written by applying the scan pulse to the first electrodes (scan electrodes) in sequence and at the same time applying the write pulse of the opposite polarity selectively to the third electrodes (address electrodes), in the write period. Following this, a voltage is applied between the first electrodes (scan electrodes) and the second electrodes (sustain electrodes) to sustain a discharge in the sustain period. As a result, an image is displayed.
Here, the base pulse which is applied to the scan electrodes is in principle of the same polarity as the scan pulse. Accordingly, even when the potential difference between the scan and write pulses is smaller than a write discharge firing voltage, if the sum of the potential difference and the base voltage exceeds the write discharge firing voltage, the voltage between the scan and address electrodes exceeds the write discharge firing voltage when the scan and write pulses are applied. Hence the write discharge takes place reliably.
The write discharge firing voltage referred to here is a voltage at which the write discharge starts in the write period.
In general, the wave height of the base pulse is substantially constant throughout the write period, but the wave height may vary within an extent that ensures the reliable write discharge, after the write discharge firing voltage is exceeded.
An explanation is given below, with regard to the average rate of change of voltage in the period from when the application of the base pulse starts until immediately before the application of the scan pulse starts.
The application of the base pulse starts at a point where the leading edge of the base pulse begins (in this specification, xe2x80x9cleading edgexe2x80x9d means a pulse portion that first increases in voltage in the case where the pulse is of positive polarity, and a pulse portion that first decreases in voltage in the case where the pulse is of negative polarity).
If the potential difference between the scan and write pulses is below the write discharge firing voltage, the sum of the potential difference and the voltage between the scan and address electrodes is smaller than the write discharge firing voltage when the application of the base pulse starts. The sum, however, increases with time, and eventually reaches the write discharge firing voltage at some point. Therefore, a voltage, which is sufficient for the above sum to exceed the write discharge firing voltage, needs to be applied between the scan and address electrodes before the application of the scan pulse begins.
Here, the average rate of change of voltage is set at 10V/xcexcsec or below so that the voltage varies gradually, in the period from when the application of the base pulse begins (base pulse start point) until immediately before the application of the scan pulse begins. This delivers the following effects.
The inventors of the present invention examined the cause of discharge errors which occur at the beginning of the write period when the absolute value of the base voltage is set high, and reached the following conclusion. At the base pulse start point, the voltage between the scan and sustain electrodes exceeds the firing voltage while there is no discharge occurring between the address and scan electrodes. This causes a large discharge.
The inventors also found that even when the absolute value of the base voltage is high, if the voltage change after the base pulse start point is gradual, only a small discharge takes place after the voltage in a discharge cell exceeds the firing voltage, and there is no occurrence of a large discharge.
Thus, according to the invention, even when the absolute value of the base voltage is high, no discharge errors occur at the base pulse start point, which benefits reliable writing of data.
If a large discharge occurs at the base pulse start point, the contrast drops due to light emission associated with the discharge. According to the invention, however, such light emission is suppressed, so that the contrast is kept from dropping.
The effects of the invention can be enhanced when combined with the set-up pulse applying technique.
Which is to say, when the base pulse of the opposite polarity is applied in the write period after the set-up pulse is applied in the set-up period, discharge errors are more likely to occur in the base pulse start point. However, by making the voltage change in the base pulse start point gradual, such discharge errors are prevented, with it being possible to achieve greater effects.
In this case, it is preferable that the average voltage change rate of the leading and trailing edges of the set-up pulse is 10V/xcexcsec or below. Also, it is preferable that the voltage continuously changes from the trailing edge of the set-up pulse through to the base pulse start point.
With the present invention, gas discharge panels which are conventionally difficult to drive, such as a gas discharge panel whose discharge gas filling pressure is no smaller than an atmospheric pressure and a gas discharge panel whose partial pressure of Xe in the discharge gas is no smaller than 10%, can be driven unfailingly.